While on all current chips, exticr gpio port mux selection is coded on 4 bits,
stm32g0 EXTI_EXTICR register uses 8 bits. Align all exti header to reference
that value (was previously defined for f0 as SYCFG_EXTICR_SKIP)
to remove variations, redundancies, add missing, fix errors. All c files
refer only to the dispatch style headers in /include/stm32. Those headers
#include memorymap.h and cm3/common.h. All references to
these are removed from the family specific headers. Ethernet untouched as
it appears incomplete.
Added dummy spi.c for F0/F3. Fix some doxygen anomalies.
So that the navigation pane works correctly in browsers.
Some additional doc fixes put in where found (but many more still to go).
Added some dummy .c and .h files to bring the associated docs into line.
makefile changed to allow 'make html' as well as 'make doc' (the latter only does html anyway).
